1. Field of the Disclosure
The present disclosure relates to a switch monitoring circuit, and more particularly to a switch monitoring circuit suitable for monitoring the state of a switch used to drive a device mounted on a vehicle, called a vehicle-mounted device.
2. Description of the Related Art
With vehicles such as automobiles, even if the ignition is turned off, an operation may be performed; for example, a window is opened or closed, a door mirror is opened or closed, and a door is locked or unlocked. To allow for this situation, a switch monitoring circuit has been used that monitors the state of a switch used to drive a power window device, a door mirror opening/closing device, a door locking device, or another vehicle-mounted device and notifies a vehicle-mounted engine control unit (ECU) and other electronic units of the monitored switch state.
Conventional switch monitoring circuits are disclosed in Japanese Unexamined Patent Application Publication No. 2005-160166 and other documents. The structure of a conventional switch monitoring circuit will be described below with reference to FIG. 6. FIG. 6 illustrates the structure of a conventional switch monitoring circuit 101.
The conventional switch monitoring circuit 101 disclosed in Japanese Unexamined Patent Application Publication No. 2005-160166 is connected to a switch 110 as illustrated in FIG. 6. The switch 110 is used to drive a vehicle-mounted device (not illustrated) such as a door mirror opening/closing device in response to a manipulation performed by a manipulator. The switch monitoring circuit 101 has a capacitor 112, a voltage applying means 120, a control means including a microcomputer 122 and the like, a voltage detecting means including an input line 124 and the like, and a diode 126.
The voltage applying means 120 is a switch monitoring circuit including a transistor 114, a transistor 116, a battery 118, and the like. The voltage applying means 120 is connected to the switch 110 through wires, connectors, and other elements, which are not illustrated. Upon the receipt of a control signal, called a strobe pulse, from the microcomputer 122, the voltage applying means 120 is activated and applies a voltage to the switch 110.
The switch 110 is a two-terminal switch. One of the terminals of the switch 110 is connected to the voltage applying means 120 through the diode 126, and the other terminal is grounded. While the voltage applying means 120 is operating and the switch 110 is open, the one terminal of the switch 110 is at a predetermined voltage. Otherwise, the terminal is at 0 V.
The microcomputer 122 periodically transmits a control signal to the voltage applying means 120 so that the voltage applying means 120 intermittently operates. The microcomputer 122 monitors the voltage at the one terminal of the switch 110 through the input line 124 and periodically monitors the open/closed state of the switch 110 according to the monitored voltage. The microcomputer 122 is connected to the electronic units described above through wires and other elements, which are not illustrated. The microcomputer 122 notifies the vehicle-mounted ECU and other electronic units of information about the monitored open/closed state of the switch 110.
For a vehicle in which this type of switch monitoring circuit is mounted, there is a constant demand for reducing a current draw on the vehicle side while the ignition is turned off (this current draw is called a dark current) to, for example, assure that charges remain in the battery of the vehicle. To maintain the conductivity of the contacts of a switch and connector for use in a vehicle, for example, there has been a need for flowing the minimum current equal to or higher than a predetermined value between the switch monitoring circuit and the switch.
With a circuit such as the switch monitoring circuit 101 described above, however, the voltage applying means 120 is periodically activated regardless of whether the switch 110 is open or closed and a current draw involved in the operation of the voltage applying means 120 becomes a dark current, which has been a factor that hinders the reduction of the dark current. If the current draw of the voltage applying means 120 is suppressed to reduce the dark current, a current flowing between the switch monitoring circuit 101 and the switch 110 is lessened accordingly. This has led to the possibility that the minimum current between the switch monitoring circuit 101 and the switch 110 cannot be maintained.